Larry Bowers, a scientist with the U.S. Anti-Doping Agency, was the government's third witness in the perjury trial of Bonds, who is accused of lying under oath when he denied using banned drugs from the BALCO steroids lab.

In 2003, Bonds told a grand jury that he had never knowingly used steroids, only items that his weight trainer identified as flaxseed oil and arthritis balm. He has pleaded not guilty to five felony charges.

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On the witness stand, Bowers never mentioned Bonds' name, but his testimony was critical to the government's effort to prove its case against baseball's home run champion.

Ex-girlfriend to testify

Prosecutors told U.S. District Judge Susan Illston that one of their next witnesses will be Kimberly Bell, Bonds' former girlfriend.

Bell has told investigators that in 1999, Bonds admitted to her that he was using steroids. After that, by her account, Bonds' body and psyche underwent dramatic changes: He became significantly more muscular, suffered occasional bouts of impotence and flareups of back acne, and had bursts of violent temper.

Other witnesses will testify that Bonds' hat and shoe sizes increased during his years with the Giants, prosecutor Matt Parrella said in his opening statement.

The judge ruled that none of that testimony could be presented to the jury unless it had a scientific underpinning. Providing it fell to Bowers, a steroid expert who works for the agency that administers drug tests to U.S. Olympic athletes.

By Bowers' account, athletes who use steroids to get bigger and stronger are prone to side effects because they tend to take the drugs in dosages that are as much as 100 times greater than what a doctor would prescribe.

Physical changes

At that level, he said steroids cause a litany of problems. Acne, especially on the back and chest, occurs because steroids stimulate sweat glands. The drugs also affect hair follicles, sometimes causing premature baldness, he said.

Sexual problems occur because the drugs can interfere with the body's ability to produce natural testosterone; that can cause occasional impotence, the scientist said. Meanwhile, he said studies by a Harvard professor have documented increases in aggressive behavior, also known as " 'roid rage."

Bowers said abuse of human growth hormone can ape the symptoms of the pituitary disorder called acromegaly. In adults, the extremities begin growing, and an "increase in shoe, ring and hat size are commonly reported," he said, quoting from a scientific paper.

The expert acknowledged there were relatively few scientific studies to support his opinion. Authorities simply won't permit scientists to dose human subjects with dangerous drugs to find out the harm they will cause, he said.

East German records

Much of what is known about the side effects comes from records of the former nation of East Germany, where during the Cold War government scientists dosed Olympic athletes with high levels of steroids, he said.

Bowers also acknowledged that there are no studies to verify that growth hormone causes acromegaly in the way a tumor-ridden pituitary gland does. But he said that's what scientists presume.

That admission led Bonds' lawyer, Allen Ruby, to ask the judge to throw out Bowers' testimony about growth hormone's alleged side effects.

"As the law tells us, unsupported theories don't get to be used in court," Ruby said.

But the judge ruled the jury of eight women and four men could still consider the testimony.

Ruby questioned Bowers aggressively, at one point querying him on a reported symptom of acromegaly that Bonds obviously never suffered: weakness in the hands. Not all symptoms appear in every case, the expert replied.